PLATE XXTIT. 



TUBULARIA INDIVISA AND TuBULARIA LaRYNX. AnATOMY AND DEVELOPMENT. 

 Fig. 



1. 2\i./mlaria indivisa, \ii\-\.\cvi\ section through the body of the hydrniith ; «, endodenn liiiiiio- 



the distal portion of tlic hydranth cavity ; h, endoderin lining the pro.\inial portion ; c, 

 pendulous lobes of endoderm separating the distal from the proximal portion of the 

 hydranth-cavity. (See page 124.) 



2. Pendulous endodcrmal lobes in hydranth-cavity of Tuhularia indivim ; profde view. 



3. A single lobe more magnified. 



4. Tuhidaria indivisa, one of the tentacles of the distal circlet, focalised in a longitudinal plane 



passing through the axis, and showing the ectoderm, endoderm, and intervening fibril- 

 lated layer. (See page 200.) 



5. Tiihihtria indivisa ; portion of one of the proximal zone of tentacles, showing the fihrillse of 



the muscular layer. In the ectoderm some thread cells are seen witli the filament 

 exserted. 

 0. Some of the fibrilltC of the muscular layer isolated and mngnitied about 200 diameters. 

 (See p. 206.) 



7. 7'«^«/ffn'« zV«/zV/««, transverse section of the stem; «, perisarc showing numerous successive 



layers of thickening ; b, ectoderai ; c, peripheral layer of endoderm composed of small 

 spherical cells with coloured granular contents, and excavated by the longitudinal ciliated 

 canals ; d, central portion of the ectoderm composed of large cells with clear, colourless 

 contents, and forming a solid, pith-like axis. (See p. 206.) 



8. Tuhularia indivisa, male gonophore; a, ectotheca ; ^, mesotlieca ; c, radiating canal; d, 



circular canal ; e, endotheca ; /, spermatic mass ; immediately over the circular canal is 

 seen the apical orifice of the gonophore. (See page 207.) 



9. Free spermatozoa of Tuhularia indivisa. 



10. Tuhularia indivisa, transverse section through spadix, showing its ciliated cavity and its 



proper tissue surrounded by the spermatic plasma. (See page 207.) 

 11 — 10. Development of actinula in Tuhularia indivisa. (See page 90.) 



11. Female gonophore; r/, ectotheca ; h, mesotheca ; c, radiating canal; d, apical orifice sur- 



rounded by circular canal ; /, generative plasma enveloping the spadix ; y, mass detached 

 from generative plasma, and about to become developed into an actinula. The endotheca 

 has already disappeared. 



12. Cells composing the mass detached from the generative plasma {(/ in fig. 11). 



13. The actinula in a very early stage. It is in the form of a concavo-convex disc, which 



embraces the spadix and residual plasma. Its interior is hollowed out into a cavity. 



14. An ectoderm and endoderm has become differentiated, and the margin presents a shallow 



lobing, the commencement of tentacles. 



15. The tentacles have become distinct, having increased in length, and the embryo now presents 



the form of a star. 



16. The actinula is fully formed and ready to escape from the gonophore; a circle of short 



tentacles, a, has become develojied round the mouth. 



17. Thread-cells from the hydranth of Tuhularia indivisa; a, ordinary form of thread-cell before 



evolution ; h, the same after evolution ; c, minute bean-shaped thread-cells. 



IS. Cells from the lining of the hydranth cavity in Tuhularia larynx. Some of these cells 

 contain within them clear secondary cells ; in others the secondary cells contain a brood of 

 still younger cells, or are filled with a coloured granular matter. Some of the secondary 

 cells with their granular contents are seen in the figure to have been liberated by the 

 rupture of the mother-cells. (See ])age 134.) 



19 — 24. Development of actinula in Tuhularia larynx. (See page 91.) 



19. Very young female gonophore; a, ectotheca; h, endotheca; /, generative plasma. 



20. Cells composing the generative plasma in fig. 19 treated with acetic acid, and magnified 



about 250 diameters. 



21. Gonophore at a more advanced stage than fig. 19; a, ectotheca; d, commencement of apical 



processes ; h, endotheca ; /, generative plasma. 



22. Cells composing the generative plasma in fig. 21 treated with acetic acid. 



23. Gonophore still more advanced; the apical processes have become larger, the endotheca has 



disappeared, and the generative plasma has thrown off a mass which is about to become 

 developed into an actinula ; /, residual plasma enveloping the spadix ; g, mass which had 

 been detached from the ])lasma. 



24. Gonophore containing a fully developed actinula in the act of liberation ; /, residual plasma 



enveloping the spadix ; y, a mass which had been detached from the plasma, lying free in 

 the cavity of the gonophore ; li, h, completely formed actinula escaping through the 

 orifice between the apical processes of the gonophore. 



